'Sneak peek', or worse still, 'sneak peak'. Just because it rhymes doesn't make it true - this always means 'carefully orchestrated early press release'.
'Naughty but nice' - I'm not going to spoil something nice by having negative thoughts about it, thank you. They should just say 'has chocolate and/or cream in it'. Or Bailey's.
Related phrase: 'Go on, spoil yourself'. What are you spoiling by enjoying something, often something with cream or chocolate. Or Bailey's. If the other person really thinks you're about to break the scales or succumb to liver disease, they shouldn't offer it to you. What they really mean is that if you refuse it, you will feel guilty if they have whatever it is themselves.